Five individual male matings of line UNH 105 New Hampshires, in which all males and most females were either B22/B24 or B22/B26, produced 462 progeny that fell into six B complex genotypes: B22/B22, B24/B24, B26/B26, B22/B24, B22/B26, and B24/B26. The genotypes of parents and offspring were determined by blood typing for B alloantigens using a panel of antisera. Six-week-old chickens were inoculated with Rous sarcoma virus (RSV). Resulting tumors were scored for size six times over a 10-week period; based upon these scores, a tumor profile index (TPI) was assigned to each chicken as a criterion of immunological response. The B22/B26 hosts showed the greatest mean response (TPI 3.3) and B24/B24 chickens the lowest response (TPI 4.4), the difference being statistically significant. Dominance in the response to sarcoma was observed when either the B22 or B26 haplotype combined with the B24 haplotype and compared with the appropriate corresponding homozygotes, and when the B22 or B26 heterozygote was compared with B22/B22 and B26/B26 homozygotes.